Previously such primer compositions have included two sensitive primary explosive constituents together with oxidizers, friction agents and fuel. The most commonly used main explosive constituent is lead styphnate which is always accompanied by a second primary explosive, tetrazene, which is essential to render the composition sufficiently sensitive to percussion and reproducible in its response. The most commonly used oxidizing agent is barium nitrate and antimony sulphide is often used as a fuel.
These primer compositions typically include elements such as lead, antimony and barium which are now considered to have high toxicity. Such elements produce a potential health hazard particularly within enclosed shooting ranges where they accumulate in the atmosphere and on surfaces. Thus, in recent years there has been a tendency towards so called non-toxic primer compositions.
These non-toxic priming compositions include diazodinitrophenol (DDNP) along with a sensitizer explosive such as tetrazene. Examples of such non-toxic priming compositions can be found in EP Patent Number 0440873 (Blount Inc), U.S. Pat. No. 4,674,409 (Olin Corporation) and others. Typically, the completeness of ignition for these compositions in particular has been unsatisfactory for rimfire ammunition. In the relatively unconfined conditions found in rimfire priming, complete and rapid explosion of the priming charge does not readily occur.
Furthermore, the necessity for a second primary explosive such as tetrazene which acts as a sensitizer within the primer composition leads to extra compounding and manufacturing requirements. In addition the inherent colour of DDNP is such that it is difficult to see within a brass ammunition casing.
It would be advantageous to establish a non-toxic primer composition that comprises a single explosive with associated friction agent, oxidizer and binder. Furthermore, if the composition has colour readily distinguishable from a brass casing there would be significant benefits. For the explosive to be non-toxic it should not contain toxic elements. For instance it could be a salt of a non-toxic element. Known non-toxic explosives formed of non-toxic elements are metal salts of dinitrobenzofuroxan including the potassium and the sodium salts (KDNBF and NaDNBF).
It is imperative that rimfire ammunition is suitably sensitive to enable consistent and reliable ignition. Normally primed cartridge cases are tested by dropping a known weight from a known height on to a striker pin abutting the rim of the case which is filled with priming composition. It will thus be appreciated that when the weight is dropped on the striker the rim is indented and the priming composition is exploded. The mean fire height is the height from which the weight must be dropped in order to explode 50% of the sample of primers. Acceptable fire heights vary for different types of ammunition and to a large extent it is more important that the priming composition is consistent in its response to percussion rather than being too sensitive.
The all-fire height is that from which the weight must be dropped to explode all primers in a sample (typically 50 primers). Typically, using a weight of 2 oz (57 g) and a chisel striker which represents that found in many target weapons, an all-fire height of between 9-11" (229-279 mm) is acceptable for rimfire target ammunition.
The primer composition must explode rapidly and completely when the rim of the cartridge is suitably struck. As shown in EP 0529230 (Blount Inc) and U.S. Pat. No. 4,674,409 (Olin Corporation) it is possible to provide a packing above the primer composition in the rim of such ammunition in order to at least initially confine the primer composition to ensure complete combustion and thus enhance the effect of its ignition of the propellant. Such confinement requires the introduction into the cartridge of combustible bodies such as a layer of compressed propellant which may interfere with the consistency of the propelling impulse given to the bullet, rendering such ammunition unsuitable for competition shooting.
Unfortunately, previous studies as exemplified by EP 0580486 (NCS Pyrotechnic et Technologies) have indicated that to be effective a combination of two primary explosives is required when using KDNBF. Tetrazene must be used as a sensitizer. Thus, single explosive primer compositions have previously been found to be either too inconsistent or too insensitive.
Additional sensitizing explosives add to manufacturing complexity and cost. Furthermore, the sensitizing explosive tetrazene is unstable and may deteriorate with time and so affect the performance of the primer.
Typically, primer compositions are mixed as a paste with a little water. This paste is then spread and packed into holes provided in a priming plate. The cylinder of primer composition packed into each hole represents the amount of primer for each casing or percussion cap.
The paste is ejected from the priming plate as a pellet into each case or cap by a suitable rod or peg. Thus, at the bottom of each casing there will be a cylinder shaped volume of primer composition paste. With rimfire ammunition it is required to pack this primer composition uniformly into the rim at the base of the case. A common technique is to spin a tool inserted within the casing such that the primer is packed into the rim. The primer is effectively squeezed or pumped into the rim.
Once the primer composition is located in the rim then each casing is either force-dried or naturally dried to drive off residual water within the composition leaving a dry fillet of sensitive composition around the base of the case. If required a small proportion of a water-soluble adhesive or binder may be included in the mix to ensure the integrity of the fillet. In the table, dry proportions are quoted for comparison between compositions.